This invention relates generally to a method and apparatus for data compression and decompression and, in particular, to a novel data compression and decompression method and apparatus for reducing the number of bits of information when transmitting a video image.
There are a number of types of apparatus and methods in existence for the compressing of digital data. One technique which has proven useful in compacting digital image data is run length encoding. This particular technique consists of partitioning each row (scan line) of the image or array into a sequence of runs, with each run including one or more adjacent picture elements, called pixels, with the same binary value, and representing each run with a symbol or code which specifies the length of the run. Often, these symbols are represented by variable length codes, such as a Huffman code set, and are assigned such that the shorter code words are used to represent the symbols which are more likely to occur, and the longer code words stand for symbols which rarely appear.
Another feature of data compression which is important is reversibility of the process. A process which is reversible may be described as one which not only reduces the amount of data of a facsimile image before transmission but also enables the reconstruction of the original image without losing the information content of that data, whereas an irreversible process is one in which a certain portion of the data is irreversibly lost during data compression. A reversible process is preferred, since it will provide a more accurate image after decompression of the image data.
Another factor which is significant to data compression efficiency concerns data at the end of a scan line. Many compression methods require an end of line character to aid in the proper reconstruction of the compressed data at the decompression end. However, if a continuous encoding method is employed, compression can be continued from line to line, which will improve the compression ratio considerably.